1. Field of the Invention
The present invention relates to a photo-sensing device, a photosensor and a display device.
2. Description of the Related Art
Thin-film phototransistors featuring thin-film transistors (TFTs) are well known.
FIGS. 17A and 17B show a typical structure of a conventional thin-film phototransistor 80. FIG. 17A is a perspective plan view illustrating the structure of a conventional thin-film phototransistor 80. FIG. 17B is a cross-sectional view of the phototransistor in FIG. 17A taken along a line D-D. The thin-film phototransistor 80, which has the same structure as a TFT installed in the pixel display part of a TFT liquid crystal display, is configured as an amorphous silicon (a-Si) TFT. The thin-film phototransistor 80 includes a substrate 81 made of glass or other materials, a gate electrode 82 formed on the upper face of the substrate 81, a gate insulating film 83 formed on top of the substrate 81 and covering the upper face of the gate electrode 82, a photoelectric conversion semiconductor thin film 84, which is provided on the central area of the gate insulating film 83 and serves as an optical sensing layer and a channel, a channel passivation film 85 patterned on the central area of the photoelectric conversion semiconductor thin film 84, a source electrode 87 and a drain electrode 88 made of a light-blocking conductive material and formed via a first and a second ohmic contact layers 86a and 86b on both sides of the channel passivation film 85, and an overcoat film 89 formed over the entire surface including the source electrode 87, the drain electrode 88 and the channel passivation film 85.
FIG. 18 is a typical circuit diagram of an amplifier using the thin-film phototransistor 80 as shown in FIG. 17A, namely a photosensor. The photosensor 92 is a two-stage amplification circuit comprising two operational amplifiers OP1 and OP2. The thin-film phototransistor 80 is connected to the noninverting input terminal of the operational amplifier OP1. The operational amplifier OP1, which includes a feedback resistor R1 connected between the noninverting input terminal and the output terminal of the operational amplifier OP1, converts electric current into voltage (i.e., current-voltage conversion). The operational amplifier OP2, which includes an input resistor R2 connected between the noninverting input terminal and the output terminal of the operational amplifier OP1, and a feedback resistor R3 connected between the noninverting input terminal and the output terminal, amplifies voltage. A peak detector comprising two operational amplifiers OP1 and OP2 is disclosed in Japanese Patent Laid Open Application, JP2000-46876 A, for example.
FIG. 19A is a schematic cross-sectional view illustrating the operation of the thin-film phototransistor 80, and FIG. 19B is an equivalent circuit diagram of the thin-film phototransistor 80. A negative DC voltage, −10 V for example, is applied to minimize dark current (output during unexposed periods), thus increasing the ratio of light current to dark current (i.e., light current/dark current ratio). A positive or negative constant DC voltage is applied between the source electrode 87 and the drain electrode 88.
In the thin-film phototransistor 80 as shown in FIG. 19A, holes are induced in the region near the interface between the channel and the gate insulating film 83 by the application of negative gate voltage (Vg). Namely, the intrinsic region of the a-Si photoelectric conversion semiconductor thin film 84, which serves as a channel, is inverted to a p-type semiconductor due to the electric field effect. Consequently, a structure similar to the p-i-n structure (pseudo-pin structure) is formed near the edge of the drain electrode 88. Likewise, the pseudo-pin structure is also formed on the side of the source electrode 87. Specifically, the thin-film phototransistor 80, when the negative gate voltage Vg is applied, is expressed as an equivalent circuit in which a pair of pin diodes is connected in series so that their rectification directions face opposite to each other as shown in FIG. 19B.
In the thin-film phototransistor 80, the current is flown from the drain electrode 88 to the source electrode 87 via the photoelectric conversion semiconductor thin film 84. If the incident light L is irradiated as shown in FIG. 19A, the absolute value of the current increases in proportion to the intensity of the incident light L entering the photoelectric conversion semiconductor thin film 84. Consequently, by converting this current, lout, into voltage and amplifying the voltage with operational amplifiers OP1 and OP2 of the photosensor 92, the output voltage of the photosensor 92 turns into an illumination signal corresponding to the intensity of irradiation of the incident light L.
The thin-film phototransistor 80, which has the same structure as a TFT installed in the pixel display of a so-called TFT liquid crystal display as mentioned above, is formed through the same process as the TFT. Consequently, the thin-film phototransistor 80 can be integrated into a TFT liquid crystal display device easily and at low cost. Therefore, a light control function can be integrated by using the thin-film phototransistor 80 in such a display device. Unlike conventional pin-type photodiodes, the thin-film phototransistor 80 having the same structure as the TFT does not require a p-type semiconductor. The thin-film phototransistor 80 can thus be configured using a semiconductor material incapable of forming a p-type semiconductor such as a-Si. Therefore, the thin-film phototransistor 80 is expected to find widespread applications.
FIG. 20 is a schematic plan view of a typical display device 100 equipped with a conventional thin-film phototransistor 80. The display device 100 comprises multiple gate lines 103 provided in parallel to each other and extending in the horizontal direction on the surface of a substrate 102, multiple data lines 104 provided in parallel to each other and extending in the vertical direction, pixel display part 105 made up of multiple pixels formed at each cross-sectional area of the gate line 103 and the data line 104, and a thin-film phototransistor 80 provided at the periphery of the substrate 102, at the lower left corner in the case of FIG. 20. One end of each gate line 103, the left end in the case of FIG. 20, is connected to a gate driver 106, which supplies gate signals sequentially. One end of each data line, the upper end in the case of FIG. 20, is connected to a data driver 107, which supplies data signals sequentially.
As shown in FIGS. 17A and 17B, the thin-film phototransistor 80 has the same structure as the TFT of each pixel display part 105, and is formed on a substrate 102 simultaneously with the TFT. As shown in FIG. 20, bias voltages are applied to the gate electrode 82, the source electrode 87, and the drain electrode 88 of the thin-film phototransistor 80 from outside the display device 100. The thin-film phototransistor 80 outputs the output current, lout, corresponding to the intensity of the incident light L. Consequently, by converting this output current, lout, into voltage using the photosensor 92 as shown in FIG. 18, and then amplifying the voltage, illumination signals corresponding to the intensity of the incident light L can be obtained. By performing drive control of the backlight of the display device 100 based on these illumination signals, the illumination by the backlight having the light intensity corresponding to the brightness of surrounding area can be obtained, and eye-friendly display can thus be ensured.
In the conventional thin-film phototransistor 80, the source electrode 87 and the drain electrode 88 partially cover the channel passivation film 85, which also serves as an etching stopper, thus blocking the light coming into the photoelectric conversion semiconductor thin film 84 and preventing sufficient amount of photocurrent from being obtained.
A back-channel etching type thin-film phototransistor 80 provides larger photocurrent than that of the etching stopper type, because there is no undesirable light-blocking structure, the photoelectric conversion semiconductor thin film 84 is several times thicker than that of the etching stopper type, short channel length allows high-density packaging and others.
However, in the conventional thin-film phototransistor 80, the pseudo-pin junction is formed when the negative gate voltage Vg is applied to the region near the interface between the photoelectric conversion semiconductor thin film 84 and the gate insulating film 83 and the interface is inverted to a p-type semiconductor because of the electric field effect as mentioned above. Such pseudo-pin junction region is formed only within a limited area near the drain electrode 88. Furthermore, since the pin diode faces the forward direction on the side of the source electrode 87, it does not serve as a thin-film phototransistor 80. Such limitations are common to thin-film phototransistors 80, regardless of the type such as backchannel etching type and etching stopper type. The photocurrent obtained through a conventional thin-film phototransistor 80 is limited, which is why it is difficult to detect low illuminance accurately.